I have a PSU on the bench and according to the official spec sheet its fan has HDB (Hydro Dynamic Bearing). Once I opened the unit I noticed that the fan is a plain Young Lin Tech which has this model number DFS132512M and according to its maker it has sleeve bearings. So what's going on?

Hm yeap it is not sealed indeed. I posted more photos of the fan.
This shaft also doesn't have spiral curves but straight ones so it isn't riffle bearing. Hysint bearing isn't supposed to be a riffle variant one?

It's supposed to be an upgrade over a traditional rifle design, but cheaper than an FDB/HDB fan.

Like I said, it's got more in common with a rifle bearing, but it's not a rifle bearing.

There's a lot of these "sleeve 2.0" designs out there, where they make minor tweaks to a sleeve or rifle design, then give it a fancy name. It's just easier to group them with the design they most closely resemble.

There's only a limited number of guys making FDB fans.
Delta, Panaflo Protechnic, & ADDA are the big ones.

I think there may be two guys who I'm forgetting, but those 4 are the big ones who get rebranded into the market the most.

I just wish YLT would use DFH as the part number for the fan, instead of DFS. As that would keep confusion down.
Though, I also wish certain companies (Cougar & Cooler Master specifically) would stop calling their fans HDB / FDB when they're not.